Technical Field
The present invention relates to catalysts for selective reduction of nitrogen oxides, and more particularly to catalysts for removal of nitrogen oxides that have enhancing effects on the reduction activity of nitrogen oxides at low temperatures and on the sulfur poisoning resistance.
Background Art
Nitrogen oxides (NOX) are usually produced when fuels are combusted, and are exhausted from moving sources such as a motor vehicle and fixed sources such as a power plant or an incinerator. These nitrogen compounds are identified as the major causes of acid rain and smog formation. Since environmental protection regulations have become stricter recently, more studies are being carried out, in response, in order to reduce nitrogen compounds through catalysts.
As a method of removing nitrogen compounds that were emitted from fixed sources, selective catalytic reduction (SCR) device that uses vanadium oxides (V2O5) as active materials impregnated on titanium oxide carriers have been generally used. Ammonia has been known as a most suitable reduction agent for the system.
However, for the titanium-type SCR catalysts that use ammonia as a reductant, a catalyst that operate under 300° C. is frequently required according to the working condition. Additionally, in case of a flue gas which contains sulfur oxides that easily poison the catalysts at low temperatures, catalysts that could with this problem also need to be developed.
For the V2O5/TiO2 SCR catalyst, high catalytic de NOX activity is exhibited at 300° C. or higher. Therefore, it is necessary to develop a catalyst which shows high activity at a lower reaction temperature. Generally, when titanium oxide (TiO2) supporters and vanadium (V) are used as active catalytic materials, additional amount of vanadium is added to increase the catalytic activity at 300° C. or lower. However, when the amount of vanadium is increased, the oxidation of sulfur dioxide (SO2) that are contained in the exhaust gas to sulfur trioxide (SO3) is induced, which then react with slipped ammonia. As a result, ammonium bisulfate, NH4HSO4 which is a solid salt, is formed.
The produced ammonium bisulfate salts are imbedded into the surfaces of the catalysts, thereby interfering with the reduction reaction. As a result, as the amount of unreacted ammonia increases, formation of sulfur trioxides (SO3) is promoted, thereby accelerating the sulfur poisoning, which eventually shorten the life of the catalysts.
Therefore, catalysts that can improve catalytic activity at low temperatures without promoting the oxidation of sulfur dioxides have been developed. In general, in order to enhance low temperature activity and sulfur poisoning resistance, tungsten has been added to vanadium/titania catalysts as a promoter. For example, when tungsten oxides were added, sulfur poisoning resistance at low temperatures could be increased.
However, since the amount of tungsten oxides used is high, approximately between 5 wt. % and 10 wt. %, the increase in the price of catalysts is unavoidable.
Moreover, most of the conventional catalysts for removal of nitrogen oxides with less sulfur poisoning have been developed such that a carrier is impregnated with special active materials.
Conventional art uses a TiO2 carrier impregnated with vanadium sulfate (VSO4), vanadyl sulfate (VO SO4) and the like, and is reacted at the range of temperatures at 300-520°. However, the problem of the previously-explained sulfur poisoning also arises in this case due to the usage of vanadium.
According to another conventional art, TiO2 carrier impregnated with active materials such as V2O5, MoO3, WO3, Fe2O3, CuSO4, VOSO4, SnO2, Mn2O3, Mn3O4 are used. However, not only the problem of the sulfur poisoning from vanadium oxides still exists, but also, the previously-mentioned high cost problem due to the usage of tungsten oxides are accompanied.